You could say that this was a prelude to the battle for Belgrade in 1944. when the Partisans started there offensive towards Belgrade to team up with the Red Army.The chief objective was to disrupt railroad communications in the valleys of the Vardar and Morava rivers, and prevent Germans from withdrawing their 300,000+ forces from Greece.

The seige of Constantinople in 718. If the Byzantines lost the seige the Rum Sultanate would have taken over the balkans, Hungary and much of Poland and Germany making the area have a strong Turkish influence as well as a lot of the area becoming Muslim in faith

Another important point in Europe’s history were the two sieges of Vienna by the Osman Empire in 1529 and 1683. Both times Vienna held and the Osman attempts to advance and take Western Europe were halted.